Mössbauer-effect study of the Morin transition and atomic positions in hematite under pressure

The Mössbauer spectrum of hematite ($\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$) has been investigated over a range of pressures extending to 53 kbar and at temperatures ranging from 77 to 340 K. Particular attention has been paid to the effects of the Morin (spin-flip) transition on the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ quadrupole splitting. The anomalous reduction in the quadrupole splitting at points in $P-T$ space near the Morin transition is explained by the application of magnetic anisotropy energydensity arguments, which also yield the mean magnetic-domain size in hematite. From the low-temperature quadrupole-splitting data and knowledge of the pressure dependence of the Morin temperature rather precise information concerning the location of atoms within the unit cell can be extracted. These results are in agreement with the latest high-pressure x-ray diffraction work. In addition, the hyperfine magnetic field is found to increase discontinuously by 1.5% through the pressure-induced Morin transition, similarly to the temperature-induced transition at atmospheric pressure. By taking advantage of recent advances in high-pressure technology and consistently applying the anisotropy energy-density theory of the Morin transition due to Artman, Murphy, and Foner, we have explained all of the features of our data, as well as the anomalies in previous work.